Contents List of Figures

314 6 MACRO– AND MICRO–JETS DRIVEN BY BLACK HOLES
of light. This is an illusion that occurs when a jet moving at slightly less than the
speed of light is pointing nearly at us. Why the jet is invisible between the black
hole and 0.41 parsecs is not yet understood. (Marscher thinks that the jet expands
too fast until its pressure falls below that of the interstellar medium, which causes
a stationary shock wave to form. The shock wave then energizes electrons and
amplifies the magnetic field, which makes the jet shine from synchrotron radiation
and inverse Compton scattering. But, he admits that the evidence in favor of this is
pretty thin.) The top panel in Fig. 150 shows the X-ray flux as a function of time
measured with RXTE. The red points correspond to what we call the X-ray dips,
when the flux was low for at least 4 consecutive observations. The blue arrows show
the times when a bright spot first showed up on the VLBA radio images, about 0.1
years after each X-ray dip. The middle panel is the slope of the X-ray spectrum;
a low value means that the higher frequency X-rays are more prominent relative to
the lower frequency X-rays. The bottom panel shows the flux at 3 radio frequencies
(green is 14.5 GHz, red is 22 GHz, and blue is 43 GHz) obtained at the University
of Michigan Radio Astronomy Observatory and the Metsahovi Radio Observatory
in Finland. The black points on the bottom indicate the flux of the radio core as
seen on our 43 GHz VLBA images.
It is apparent from this Figure that each X–ray dip is followed by the appearance
of a superluminal knot at the side of the radio core. The mean time–delay between
the minimum in the X–ray flux and the appearance of the knot ejection is 0.1 ± 0.03
yr. At a typical superluminal apparent speed of 4.7 c, a knot moves a distance of
0.14 pc in 0.1 year, projected on the plane of the sky (the real distance is about 0.5
pc for an inclination of 20 degrees). The jet probably starts as a broad wind until it
gets collimated at the radio core on the scale of a few 100 Schwarzschild radii (see
M 87, Junor, Biretta and Livio, Nature 1999). this would correspond to a minimu
distance of 0.3 pc from the X-ray source to the radio core. The core of the radio
jet is therefore not coincident with the position of the Black Hole. The jet might be
invisible between the site of the origin and the radio core far downstream where the
electrons are reaccelerated (e.g. by a recollimation shock).
This correlated radio and X–ray behaviour is similar to the microquasar GRS
1915+105: here, X–ray dips, where the spectrum hardens, are followed by the ejection
of a bright superluminal radio knot. This is seen as resulting from an instability
where a piece of the inner disk breaks off. Most of this matter is accreted towards
the horizon, but a smaller amount is ejected from the inner region and accelerated
by magnetic forces to high velocities. The X–ray flux is due to Compton scattering
in the inner accretion torus. The bulk velocity of the jets in 3C 120 and GRS
1915+105 are also similar, about 0.98 c in each case (i.e. a Lorentz factor of about
5). The jet in the microquasar is however seen at a much higher inclination of 66
degrees. Therefore, the counterjet is also visible.
Nakamura et al. (2001) investigated the wiggled structure of AGN radio